Activators of Nuclear Hormone Receptors, PPARα and FXR, Accelerate Maturation of the Epidermal Permeability Barrier In Utero • 321

Immaturity of skin barrier function contributes to the mortality and morbidity of premature birth. Using the fetal rat as a model to study regulation of barrier ontogenesis, we have previously shown that: 1) formation of a competent barrier to transepidermal water loss (TEWL) is invariably accompanied by the development of a multilayered stratum corneum in which lipid is deposited in the intercellular domains to form mature, multilamellar unit structures, ultrastructurally; and 2) certain hormones, such as glucocorticoids, thyroid hormone and estrogens, accelerate barrier maturation. These hormones, however, do not appear to be required for barrier formation, because epidermal development proceeds in fetal rat skin explants cultivated in hormone-free media. Recently, we showed that activators of PPARα and FXR, when added to the culture medium, accelerate barrier formation in vitro, suggesting that epidermal differentiation and development may be regulated by endogenous ligands of nuclear hormone receptors which heterodimerize with RXR. Here, we asked whether the PPARα ligands, clofibrate or linoleic acid, or the FXR activators, farnesol or juvenile hormone III, stimulate cutaneous development in utero. PPARα and FXR activators, or vehicle (DMSO), were injected intra-amniotically in pregnant rats on gestational day 17 (term = 22 days). Pups were delivered on day 19, TEWL was measured, and epidermal morphology assessed by light and electron microscopy. Whereas vehicle-treated fetal rats displayed no measurable barrier (TEWL > 10 mg/cm²/hr), pups treated with PPARα and FXR activators exhibited barrier formation (TEWL range 4.0-6.6 mg/cm²/hr, p<0.01, n=6). Control pups lacked a well-defined stratum corneum (SC) by light microscopy, while a distinct SC, which showed increased neutral lipid staining in a membrane pattern using the fluorescent probe, nile red, was present in treated pups. Moreover, electron microscopy demonstrated a paucity of mature extracellular lamellar membranes in the SC of control pups, while these structures filled the SC interstices in treated pups. Thus, SC maturation and barrier formation in utero are accelerated by PPARα and FXR activators, suggesting a physiologic role for these receptors and their putative endogenous activators, fatty acids and farnesol, during skin development.